The present invention relates to a process for hydrolyzing lactose. More particularly, the present invention relates to a process for hydrolyzing lactose, such as that contained in whey, by means of an immobilized lactase.
In 1976, French cheese production reached approximately one million tons, resulting in the production of more than six million tons of whey as a by-product. Such whey contains, per liter, approximately 6 to 9 g. of protein, 45 to 50 g. of lactose, 6 to 8 g. of mineral salts, and 1 to 2 g. of fat. World-wide, the amount of lactose available from whey alone in 1977 totaled almost 3.5 million tons.
In the past, whey was regarded as a waste product and was discharged into sewers or streams and rivers. Because of increasing concerns over environmental pollution, much of the whey is being processed into a variety of animal and human foods. For example, the development of high performance ultrafilters now permits the separation of whey protein from the whey. Such protein is of exceptional value. For example, a 35 percent concentrate of whey protein can replace nonfat dry milk in many food products, such as baked goods, beverages, and frozen desserts. Unfortunately, the separation of proteins from whey results in a liquid fraction, called permeate, which until recently has had little or no value.
With the advent of immobilized enzymes, however, hydrolysis of the lactose in the permeate, either with or without demineralization, has become commercially feasible. Because of the presence of glucose and galactose, hydrolyzed lactose is much sweeter and more soluble than lactose alone. Thus, the hydrolyzed product is a functional sweetener which can be used in the preparation of pastries, milk-based desserts, and frozen confections such as ice cream. Furthermore, the hydrolyzed product is an efficiently fermentable mixture suitable for use as a fermentation substrate in, for example, the brewing and pharmaceutical industries.
Various methods for hydrolyzing lactose are, of course, well known to those having ordinary skill in the art. Enzymatic hydrolysis is especially useful for the production of food-related products and, as already indicated, the use of immobilized enzymes is particularly attractive.
By way of illustration only, H. H. Weetall et al., Biotechnol. Bioeng., 16, 295 (1974), discuss the preparation of immobilized lactase and its use in the enzymatic hydrolysis of acid whey. The enzyme, isolated from both fungi and yeast, was immobilized on airconia-coated porous glass particles. The substrate consisted of either aqueous lactose solution or acid whey permeate.
Additionally, L. E. Wierzbicki et al., Biotechnol. Bioeng., 16, 397 (1974), reported on the hydrolysis of lactose in acid whey using lactase (.beta.-galactosidase) immobilized on porous glass particles with emphasis on the preparation and characterization of a reusable catalyst for the production of low-lactose dairy products. Partially purified lactases from Aspergillus niger, Lactobacillus helveticus, and Saccharomyces lactis were immobilized on porous glass particles. The substrate consisted of acid whey powder which had been reconstituted in water to the appropriate solids concentration. In some instances, the reconstituted acid whey was deproteinized by heating in a boiling water bath for five minutes.
Finally, H. H. Weetall et al., Biotechnol. Bioeng., 16, 689 (1974), describe the preparation of immobilized lactase as part of continued studies on the enzymatic hydrolysis of lactose. A fungal lactase was employed, immobilized on zirconia-coated controlled-pore glass and porous titania particles. The resulting immobilized enzyme preparations were used for the hydrolysis of lactose in whole sweet whey, whole acid whey, acid whey ultrafiltrate, and pure lactose.
It also is possible to carry out the hydrolysis of the lactose in whey without a prior ultrafiltration step and either with or without demineralization. The product, which still contains proteins, is similar to hydrolyzed permeate and, consequently, it also can be used in the food industry as already described.
Unfortunately, however, the hydrolysis of lactose in whey by means of immobilized enzymes without a prior ultrafiltration step is not without problems, especially on a commercial scale.
Such problems in general are related to the presence in the whey of proteins which tend to deposit on the immobilized enzyme bed, i.e., to coat the immobilized enzyme. The protein coat in turn impedes or even stops the diffusion of lactose to the enzyme and the diffusion of the hydrolysis products, i.e., glucose and galactose, away from the enzyme. Consequently, there is, with the passage of time, an apparent decrease in enzymatic activity. The net effect is to render the use of an immobilized lactase for the hydrolysis of lactose in whole whey impractical on a large-scale, continuous basis. Furthermore, removal of the protein by ultrafiltration prior to hydrolysis tends to remove bacterial contamination, thereby reducing bacterial growth on the immobilized enzyme which contributes to the apparent decrease with time of enzymatic activity and the plugging or clogging of the immobilized enzyme reactor.
It should be apparent, though, that any economical commercial process for the hydrolysis of whole whey must maintain enzymatic activity above some minimum value for an adequate period of time. Consequently, efforts undertaken to find a satisfactory procedure for whey hydrolysis with immobilized enzymes primarily have concentrated on the use of demineralized whey permeate. Although various treatments of the immobilized enzyme bed, such as periodic washing and disinfection (with, for example, acetic acid solutions), have been proposed, there still remains a need for a commercially viable procedure for whole whey hydrolysis, i.e., hydrolysis of whole whey which has not previously been subjected to an ultrafiltration treatment.